Method and device for determining a characteristic value that is representative of the condition of a gas

ABSTRACT

The invention relates to a device and methods for characterizating flowing substances, liquid or gas. The device includes: a transport duct with a heating or a cooling element; a temperature difference sensor having a temperature measurement cell downstream of the heating or cooling element and means to determine a temperature difference in the flowing substance upstream and downstream of the heating or cooling element; a flow control means having flow measurement means for measuring a mass flow characteristic and a flow correction means for correcting for measured mass flow variations; and an evaluation means for evaluating a characterizing feature of the flowing substance comprising a function relating temperature differences measured on one or more calibration substances to one or more characterizing features of the flowing substance. Various embodiments relate to the related use and methods of the device for identification and control of the flowing substance.

This application is a 35 U.S.C. §371 application of WO 2004/036209,filed Oct. 17, 2003 claiming priority to EP Appl. No. 02023415.9 filedOct. 18, 2002, both of which are incorporated herein by reference intheir entirety.

BACKGROUND OF THE INVENTION

The invention relates to a device and methods for the characterisationof a flowing substance. Particular embodiments of the invention relateto the use of the device for the identification of a flowing substance,for controlling the flow of a fuel or combustion gas to deliver acontrolled heat of combustion and for measuring the heat capacity of agas. Further embodiments of the invention relate to a flow controldevice for controlling the flow rate of a flowing substance and a methodfor the combustion of a fuel or combustion gas.

Flow control devices are used to deliver a controlled amount of aflowing substance. This works well in most situations, where the natureof a flowing substance is known and substantially constant incomposition over time. However, there are also situations where thenature of the flowing substance is not known and can vary in time. Sucha situation occurs for example in natural gas distribution networkswhere different suppliers pump natural gas of different origin and/orcomposition into the network to various customers. The gas supplied tothe customers can vary over time in composition and heat of combustion.The flow of the gas changes in an unknown way because of changes in theviscosity, the density or heat capacity of the various different gasses.In such a situation, most flow control devices, like thermal flowcontrollers, cannot deliver an accurately controlled mass flow. Becauseof these unknown changes in the mass flow and heat of combustion,problems can occur like the incomplete combustion of the gas, a too lowenergy recovery and emission problems.

Another example of such a situation, and a new application of theinvention as described hereafter, is the distribution of fuels forengines. For optimum engine performance and fuel economy it is desirablethat the flow control is able to deal with varying and/or unknown fueltypes and compositions. This will become more and more relevant in viewof the increasing desire to further optimise fuel economy, to minimisedamaging exhaust gasses and in view of the expected increasingdiversification in fuels. Hence, there is a desire for a device for thecharacterisation of the flowing substance.

Direct and accurate measurement of combustion energy is achieved byactually combusting the gas and measuring the heat. Well known devicesfor the combustionless characterisation of flowing substances, likechromatography or spectrometry, are accurate but have the disadvantagesthat they are difficult to integrate in-line, that the response time forcharacterisation of the substance is still too long for accurate controlof the flow and that the devices are too expensive and too big for manyapplications.

DE 41 18 781 describes a device for the characterisation of a gas, inparticular for the determination of the Wobbe number and/or heat ofcombustion of a gas, comprising a pressure control, a lamellar pressureresistance for isothermal expansion of the gas and a mass and a volumeflow rate measurement unit. It is described that the gas cools a heatingwire with constant over-temperature and the heating power supplied isused as a measure for the mass flow. Apart from that, the volume flowrate is measured. Further, in the device the gas is expandedisothermally over a flow resistance with laminar flow giving rise to apressure drop. The pressure drop over the flow resistance; the mass andthe volume flow measurement unit is measured, which correlatesapproximately with the density of the gas. The Wobbe number and/or theheat of combustion are evaluated from the measured mass flow, volumeflow and pressure drop by substituting the values in an empiricalfit-formula, in which the constants are determined by calibration.

EP-A-0469649 describes a calorimeter in which a laminar flow type flowmeter for measuring volume flow is put in series with a thermal typeflow meter as described above for measuring a mass flow, in which thegas flow is controlled by controlling a constant temperature differenceover the sensor and the caloric value of the fuel gas is evaluated fromthe pressure difference over the volume flow meter. Because thiscalorimeter requires two flow meters it is rather complex, expensive andbulky. Further, the accuracy and reliability are insufficient.

FR 2818746 describes a process for the evaluation of a Wobbe parameterwherein a combustion gas is compressed and subsequently expanded andwherein on expansion the flow, pressure and temperature characteristicsare measured and wherein a reference gas is similarly measuredfor-determining reference values for accurately evaluating the Wobbeindex from an empirical fit formula.

Similarly, in EP-A-0715169 the pressure of a defined volume of gas israised to a defined pressure level and after that relaxed to theoriginal state, during which relaxation the density and/or the viscosityvalues are determined, which are input to an empirical fit formula forevaluating the caloric value and/or Wobbe index.

WO 93/08457 and EP 0554095 describe an improved apparatus and methods toderive from multiple measured characteristics of a fuel gas the desiredfuel characteristic like for example a Wobbe index.

The problem of the described devices is that they are all relativelycomplex, expensive and bulky. In particular, it is a disadvantage thatthe devices are deigned only for the evaluation of heat of combustion ofa fuel gas. More importantly, the devices are often too inaccurate andlack reproducibility for more demanding applications.

SUMMARY OF THE INVENTION

There is a desire for a relatively simple, cheap and compact device anda method for a quick but accurate and reliable characterisation of aflowing substance, gas or liquid, that can be used in a flow controldevice to control the flow rate of an unknown substance. The problem andobject of the invention is to provide such a device and method.

This problem is solved according to the invention by a device for thecharacterisation of a flowing substance comprising:

-   -   a transport duct on which is mounted a heating or a cooling        element,    -   a temperature difference sensor comprising a first temperature        measurement cell downstream of the heating or cooling element        and means to determine a temperature difference in the flowing        substance upstream and downstream of the heating or cooling        element,    -   flow control means comprising flow measurement means for        measuring a mass flow characteristic and flow correction means        for correcting for measured mass flow variations, and    -   evaluation means for evaluating a characterising feature of the        flowing substance comprising a function relating temperature        differences measured on one or more calibration substances to        one or more characterising features of the flowing substance.

The invention also provides a method for the characterisation of aflowing substance that is relatively simple but accurate. This methodcomprises the steps of

-   -   locally heating or cooling the substance flowing through a        transport duct, thus creating a temperature difference in the        flowing substance upstream and downstream of the heating or        cooling element,    -   determining the temperature difference,    -   controlling the flow by measuring a mass flow characteristic and        correcting the flow for measured mass flow variations, and    -   comparing the measured temperature difference with corresponding        temperature differences measured on calibration substances for        evaluating a characterising feature of the flowing substance.

It was found that, in the device and method according to the invention,a very accurate and reproducible temperature difference measurement isobtained and that hence a reliable characterisation can be done.

It is noted that in many publications the term mass flow isinappropriately used. For example in DE 41 18 781 the heating power ofthe heating element is proportional with the mass flow but also dependson many other factors and cannot be used directly to characterise andcontrol the mass flow of an unknown substance. In the presentapplication mass flow characteristic means, as described in more detailhereafter, a characteristic relating directly to the real mass flow ofthe unknown flowing substance.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a description of certain embodiments of the inventiongiven by way of example only and with reference to the appendeddrawings, in which:

FIG. 1 is a schematic block diagram of one embodiment of the deviceaccording to invention including measurement of the pressure differenceover the sensor; and

FIG. 2 is a schematic block diagram of another embodiment of the deviceaccording to invention including a Coriolis sensor; and

FIG. 3 is a block diagram of oxygen and a combustible gas mixing inaccordance with an illustrative embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The flowing substance is forced to flow through a transport duct (30) onwhich is mounted a heating or a cooling element (40). The stream beforethat element is referred to as upstream (indicated I in FIGS. 1 and 2),the stream after the element is referred to as downstream (indicated IIin FIGS. 1 and 2). In FIGS. 1 and 2 the flow is forced through thetransport duct by a restrictor (142), which can be solid semi-permeableor variable. The heating or cooling element locally heats or cools thesubstance flowing through a transport duct with an accurately constantamount of heat creating a temperature difference in the flowingsubstance upstream and downstream of the heating or cooling element. Thetemperature difference sensor (50) comprises a first temperaturemeasurement cell (51) downstream of the heating or cooling element. Asingle downstream measurement cell can be used if the temperatureupstream is known from another source and available to the means (52) todetermine a temperature difference in the flowing substance upstream anddownstream of the heating or cooling element. For accuracy reasons, asecond upstream measurement cell (53) is preferred to measure the actualtemperature difference. Although in principle good results can beobtained with a cooling element, for example a Peltier element, aheating element is preferred because a cooling element has the risk thatgas can condensate on the element. A heating element is more accurateand less expensive. In applications where the flowing medium is a liquidhaving a boiling point near the measurement temperature, a risk mayexists that on heating a phase transition to gas exists. In such cases aPeltier element is preferred.

Devices comprising a heating element and a temperature difference sensorcomprising an upstream and downstream temperature measurement cell areknown and used as flow controllers, in particular gas flow controllers.Such gas flow controllers are commercially available from EmersonProcess Management, Brooks Instrument Division®. The measuredtemperature difference is proportional to the amount of gas flowingthrough the sensor and can be used to measure and control the flow.

If, in a given temperature difference measurement sensor, the flow rateof the flowing substance is too high or too low, the accuracy of thetemperature difference measurement is too low. Therefore, the deviceaccording to the invention preferably also comprises a flow rate meansfor adjusting flow through the transport duct in a detection range ofthe temperature difference sensor. The flow rate means can be a valveupstream of the sensor (93) or a variable restrictor (142) in or bridgedby the transport duct or a combination of a valve and a restrictor.

The device comprises flow control means comprising flow measurementmeans (61) for measuring a mass flow characteristic and flow correctionmeans (62) for correcting for measured mass flow variations. Theinventors have found that this particular flow control means correctingfor real mass flow variations assure an accurate and reproducibletemperature difference measurement. The high accuracy andreproducibility assures that the device is capable of identifyingsubstances even if they are very similar and have very close heatcapacity.

The flow measurement means (61) can determine the mass flowcharacteristic directly, by special separate mass flow measurementdevices as described below in more detail, or indirectly by measuringthe pressure difference over the sensor, preferably at known absolutepressure and temperature. It should be noted that sometimes flow metersor controllers are called mass flow meters/controllers whereas theyactually measure/control only volume flow or even just an approximationof the volume flow. The inventors found that using the volume flowcontrol is inadequate for characterising and identification of a flowingmedium and gives unreliable results.

Flow correction means (62) can be physical or mathematical. Physicalflow correction involves a pressure difference control maintaining asubstantially constant pressure difference. Mathematical flow correctioninvolves correcting the measured temperature difference for theinfluence of measured variations in the mass flow.

The evaluation means (70), for example a computer, use the temperaturedifference measurement for evaluating a characterising feature of theflowing substance. The evaluation means comprise a function, for examplea data-base, a calibration equation or the test of a condition, relatingtemperature differences measured in defined standard conditions on oneor more calibration substances to one or more characterising features ofthe flowing substance. The evaluation means (70) return thecharacterising feature as output of the device to the user or to anotherdevice. Characterisation of the flowing substance is broadly defined.Depending on the particular application of the device characterisationcan be a quantitative measurement of the heat capacity of the substance,the qualitative identification of the substance, ancillary knowninformation of the identified substance or an empirically derived outputsignal for example for controlling another device.

The function can be a data-base that can be used as a look up table toprimarily identify the calibration substance with the best correspondingtemperature difference and subsequently return as output one or morecharacterising features of that calibration substance listed in thedatabase. The database can comprise many different characterisingfeatures. It can comprise substance features, for example the heatcapacity, the heat of combustion in case of fuels or natural gasses, butalso derived features like the name of the substance, the composition ofthe substance, set-parameters for controlling another device, an addressof the supplier of the substance etc.

The function can also be a calibration equation relating the measuredtemperature difference to a characterising feature. The equationcomprises constants determined by measuring the temperature differenceon a number of different calibration substances and fitting the resultsto the known characterising features of the calibration substances. Thischaracterisation directly evaluates a property of the substance withoutidentifying the substance first. The calibration equation can forexample relate the measured temperature difference to the specific heatcapacity, the Wobbe number of the substance.

The evaluation means can also be used to derive information not directlyrelated to the nature of the substance. In case of changing compositionsof known components the evaluation means can evaluate the compositionparameters. In application to control burners or engines the resultingoutput of the evaluation means can be, instead of the heat of combustionor Wobbe number of the gas, just the optimal control set-parameter of aburner or engine. The function is can than be determined quasi empiricalby testing optimum performance of the burner for a number of differentgasses.

The function can also be the evaluation of a condition, for examplecomparing the measured temperature difference with a threshold value andreturning by the evaluation means a yes or no or an action based on thatresult.

The function can also be a combination of the aforementioned functions.For example, the evaluation means can determine the heat of combustionby identifying the substance and returning the heat of combustion of thesubstance. In case the evaluation means cannot give an unambiguouspositive identification, the calibration function can give an estimateinstead.

The function is defined at known standard measurement conditions inorder to get a meaningful evaluation of a characterising feature from atemperature difference measured on an unknown flowing medium. Similarlythe calibration substances are measured at standard measurementconditions. Standard measurement conditions are primarily the absolutemeasurement temperature and pressure. To know the real temperaturedifference characteristic for the substances, the signal from thetemperature measurement cells (53 and 51) should further be calibratedwith known substances for the particular apparatus and chosen pressuredifference over the temperature difference sensor. The measurement ofunknown substances does not necessarily have to be performed at standardconditions if the temperature and pressure are known or sufficientlyconstant to allow for mathematical correction as explained below.

The flowing substance (20) can in principle be any substance capable offlowing through the transport duct of the sensor, for example a gas, aliquid, an emulsion or dispersion. The preferred mode of construction ofthe device differs depending on whether the flowing substance is aliquid or a gas.

In a first embodiment of the device, the flow measurement means (61)comprise a pressure measurement cell (90) for measuring a pressuredifference over the temperature difference sensor (50) and the flowcorrection means (62) comprise pressure difference control means (91)for maintaining a substantially constant pressure difference over thetemperature difference sensor, for example by operating a valve upstream(93) or downstream of the temperature difference sensor (see FIG. 1). Itwas found that very accurate and reproducible temperature differencemeasurements are obtained when the pressure difference over the sensoris maintained substantially constant.

A single pressure measurement cell upstream or downstream of the sensorcan in principle be used if the pressure at the other end is known fromanother source such that the pressure difference in the flowingsubstance upstream and downstream of the heating or cooling element canbe determined. For an accurate characterisation of the flow through thesensor, the pressure should be known directly before and after thetemperature difference sensor and hence preferably is measured as closeas possible to the sensor. Therefore, a second pressure measurement cellmeasurement cell (92) is preferred to measure the actual pressuredifference. The first and second pressure cell can also be integrated ina single pressure difference measurement cell.

In another embodiment the invention the flow measurement means (61)comprise a pressure measurement cell (90) for measuring a pressuredifference over the temperature difference sensor (50) and the flowcorrection means (62) comprise computing means for mathematicallycorrecting the measured temperature difference for a measured pressuredifference variation. The mathematical correction by the flow correctionmeans can be a multiplication of the measured temperature difference bythe ratio of the measured pressure difference and the standard pressuredifference, that is the pressure difference used for measuring thecalibration substances. In view of the accuracy of the temperaturedifference measurement, the mathematical correction is preferablyperformed on the basis of a calibration function relating the pressuredifference over the sensor to the measured temperature difference.Preferably, the absolute temperature and absolute pressure aresubstantially the same as in the calibration measurements.

In another embodiment the flow measurement means (61) comprise a massflow sensor (110) measuring the mass flow through the temperaturedifference sensor (50) and the flow correction means (62) comprise massflow control means (91) for maintaining a substantially constant massflow through the temperature difference sensor (50). In an alternativeembodiment (shown in FIG. 2), the flow correction means (62) comprisescomputing means (111) for mathematically correcting the measuredtemperature difference for a measured mass flow variation. Themathematical correction of the mass flow variation with respect to thestandard measurement condition is done as described above.

Suitable mass flow sensors (110) are, for example, a coriolis-, anultrasonic or a sonic nozzle mass flow sensor. Mass flow sensorsdirectly measure mass flow, irrespective of pressure differencevariations over the sensor and absolute temperature and pressuredifferences. A mass flow sensor allows for more accurate control of themass flow through the sensor and accurate and reproducible temperaturedifference measurement. This embodiment, using Coriolis mass flowmeasurement, is particularly suitable for characterising fluids. Afurther advantage of using the mass flow sensors described above is thatthey as a result of the same measurement also produce the density of theflowing medium which can be used as additional information by theevaluation means (70) to identify or characterise the flowing medium.The resolution of the identification, that is the chance of uniquelyidentifying an unknown compound in a set of (similar) compounds,increases by looking in the calibration substances database for theclosest match in both the measured temperature difference and thedensity.

In an open-ended system, changes in atmospheric pressure can causesignificant changes in the downstream absolute pressure. Keeping thepressure difference over the sensor constant, as in one embodiment ofthe invention, does not (necessarily) result in a constant absolutepressure in the sensor. In case of liquids the effect of absolutepressure deviations are small. In case the flowing substance is a gascorrection is however preferred. To further improve the reproducibility,the device according to the invention preferably comprises pressurecorrection means to correct for an absolute pressure variation in theflowing substance.

The correction preferably is physical because that gives more accurateresults. The correction means comprise a pressure control formaintaining a substantially constant absolute pressure in the flowingsubstance, for example by including a valve up- and down stream and aseparate control for maintaining a substantially constant absolutepressure. Mathematical correction can be applied here as well.Mathematical correction of the absolute temperature or pressure has theadvantage that the device is simpler and cheaper.

Preferably the device also comprises temperature correction means tocorrect for an absolute temperature variation in the flowing substance.The correction can be physical, for example by a thermo stated housing,or mathematical by temperature correction means comprising temperaturemeasurement means (121) and calculation means (122) for mathematicalcorrection of the measured temperature difference over the sensor (50)for a measured absolute temperature variation.

In a preferred embodiment, the flow measurement means (61) comprise apressure measurement cell (90) for measuring a pressure difference overthe temperature difference sensor (50) and the device further comprisestemperature correction means to correct for an absolute temperaturevariation in the flowing substance and pressure correction means tocorrect for absolute pressure variations in the flowing substance. Theadvantage is that this device shows a more accurate and veryreproducible temperature difference measurement. A schematicrepresentation of a preferred embodiment is illustrated in FIG. 1, thedevice comprises an upstream (92) and a downstream pressure cell (90) ofthe pressure difference measurement, wherein the downstream pressurecell is used also to determine absolute downstream pressure for input tothe mathematical correction means. The pressure difference is input tophysical flow control means (62) comprising a pressure differencecontroller operating a valve (93) to maintain substantially constantpressure difference over the temperature sensor (50). Further, in FIG. 1a temperature measurement means (121) determines the absolute downstreamtemperature for input in calculation means (122) for correction of theabsolute temperature variations.

In case the flowing substance is a gas, mathematical correction fordeviations in absolute pressure and temperature from the standardconditions can in a first approximation be calculated from a generalformula based on the ideal gas behaviour of gasses. The correctedtemperature difference value (ΔT_(meas)) is obtained by multiplying themeasured temperature difference (ΔT_(meas)) by a correction factor(Q_(T)) for absolute temperature deviations (T−T_(stand)) and by acorrection factor (Q_(P)) for absolute pressure deviations(P−P_(stand)). Q_(T) and Q_(P) are 1 in case the device has physicalcorrection for absolute temperature or pressure.

ΔT_(corr) can be calculated as follows:ΔT _(corr) =ΔT _(meas) *Q _(T) *Q _(P)Wherein:Q _(T)=1+(T−T _(stand))/(273+T _(stand))+0.00286*(T−T _(stand))Q _(P) =P _(stand) /P (P in mbar)

Preferably, the mathematical correction can be done on the basis of acalibration determined by measuring the absolute temperature or pressuredependence of the temperature difference measurement on calibrationsubstances that are similar to the unknown flowing substances expected.This can be done in a meaningful way for well-defined application areas,where the general nature of the flowing substances is known as forexample, in characterising unknown natural gasses. The absolutetemperature and pressure dependence can be determined on a set ofdifferent natural calibration gasses to establish the range and averageof the corrections factors to be applied.

Typically, the transport duct (30) is mounted as a by-pass (140) on amain duct (141) in which a part of the substance flows from the mainduct through the transport duct and sensor and back into the main duct.To create the flow through the by-pass a restrictor element (142) can beintegrated in the main duct creating a pressure difference over thetemperature difference sensor. The restrictor element can for example bea valve, a duct element with a smaller cross-section area or a poroussubstance.

In a special embodiment the device comprises a switch (160) forswitching on and off the flow control means. In the on-mode the pressurecontroller is used according to the invention to control a constantpressure difference over the sensor for characterisation of the flowingmedium. In the off-position, the actual flow rate of the flowingsubstance through the transport duct is measured, now using the measuredtemperature difference and/or using the pressure difference over thesensor as a measure for the mass flow. The flow rate can also becontrolled to a constant set value (96) by the same pressure controller(91) that controls the pressure difference over the sensor by operatingvalve (93). The advantage is that the device is more versatile becauseit can perform as flow controller and for characterising a substance.

Another preferred embodiment comprises means for intermittentlyswitching the switch to the on- and the off position and means tocontrol the flow rate of the flowing substance in the off-position ofthe switch at a level determined by a characterising feature of theflowing substance determined in the on-position of the switch. Theadvantage of this intelligent device is that it can control the flowdepending on the characterising feature or nature of the flowingsubstance. For example, the device in the on-mode identifies the gas,retrieves from the database the heat of combustion of the gas, thendetermines the flow rate required for delivering a constant heat ofcombustion according to a given preset level. This device isparticularly useful for controlling the flow of a fuel of varyingorigins and compositions to combustion engines or ovens.

The device according to the invention may be used for the identificationof a flowing substance, in particular natural gasses, or for controllingthe flow of a fuel to deliver a controlled heat of combustion, forexample to an engine or to a burner, as an analytical instrument for thedetermination of the heat capacity of a flowing substance or, forexample in environmental protection and control, for the identificationof the origin, source or supplier of a flowing substance. An enginecomprising a device for controlling fuel has the advantage of being ableto optimally combust the fuel even when the composition varies from timeto time. The device and method of the invention can be used also todetect air pollution. The database can then be adapted to enableidentification of the source and nature of the pollution.

A method for the characterisation of a flowing substance may comprisethe steps of:

-   -   Locally heating or cooling a substance flowing through a        transport duct by a heating or cooling element,    -   Determining a temperature difference in the flowing substance        upstream and downstream of the heating or cooling element,    -   Controlling mass flow of the flowing substance in the transport        duct by measuring a mass flow characteristic and correcting for        measured variations in the mass flow characteristic, and    -   comparing the measured temperature difference with corresponding        temperature differences measured on calibration substances for        evaluating a characterising feature of the flowing substance.

Detailed explanation of preferences and advantages described above forthe device correspondingly apply where appropriate to the methodsdescribed above.

The method can be carried out in many different embodiments. In oneembodiment of the method the measured mass flow characteristic is apressure difference over the heating- or cooling element and themeasured temperature difference is corrected mathematically for ameasured pressure difference variation. The advantage of this embodimentis that it is simple and cheap to produce, whilst giving sufficientlyaccurate temperature difference measurement and characterisation of theflowing substance for many purposes and applications.

In another embodiment of the method the measured mass flowcharacteristic is a pressure difference over the heating- or coolingelement and the mass flow is corrected for a measured pressuredifference variation to maintain a substantially constant pressuredifference over the element. The advantage of this embodiment is thatthe physical correction gives more accurate and reproducible temperaturedifference measurements and hence more reliable characterisation of theflowing substance.

For more critical applications, where a high reproducibility andaccuracy of the temperature difference measurement is requiredpreferably further correction is made for absolute temperaturevariations and absolute pressure variations in the flowing substance ina way described above. In a preferred embodiment, wherein the measuredmass flow characteristic is a pressure difference over the heating- orcooling element, the mass flow is corrected for the measured pressuredifference variation to maintain a substantially constant pressuredifference over the element and correction is made for absolutetemperature variations and absolute pressure variations in the flowingsubstance.

In a different embodiment of the method the measured mass flowcharacteristic is the real mass flow rate in the transport duct measuredwith a mass flow sensor and physically corrected to maintain asubstantially constant mass flow rate. The advantage of this embodimentis that the temperature difference measurement is very accurate andreproducible. Suitable mass flow sensors and their applications aredescribed above.

In a more simple and cheaper but still very accurate and reproducibleembodiment of the method the measured mass flow characteristic is a realmass flow rate in the transport duct measured with a mass flow sensorand the measured temperature difference is mathematically corrected fora measured mass flow rate variation.

In the method the flowing substance is identified by retrieving from adatabase the identity of the calibration substance with the bestcorresponding temperature difference measurement. Once the flowingsubstance is identified the known properties of that substance can bereturned to the user. In an important application of the method theflowing substance is a natural gas and the natural gas is characterisedby measuring the temperature difference and retrieving from a databaseor function, relating temperature difference measurements of differentnatural gasses with one or more characterising features of said naturalgasses, the characterising feature relating to the measured temperaturedifference measurement. For example, in the method the database orfunction comprises the heat of combustion of calibration gasses and thetemperature difference measurement is used to determine the heat ofcombustion of the gas. This principle can be used in a method for thecombustion of a combustion gas wherein the combustion gas ischaracterised according to the method of the invention and mixed with anoxygen containing gas in a mixing ratio based on the measuredtemperature difference as shown in FIG. 3.

The combustion gas can be natural gas, bio-gas or industrial wastegasses, but also the vaporised fuel of combustion engines.

Thus, the invention has been described by reference to certainembodiments. It will be recognised that these embodiments aresusceptible to many modifications and alternative forms well known tothose of skill in the art. Accordingly, although specific embodimentshave been described, these are examples only and are not limiting on thescope of the invention.

1. A device for the characterisation of a flowing substance (20)comprising: a transport duct (30) on which is mounted a heating or acooling element (40), a temperature difference sensor (50) comprising afirst temperature measurement cell (51) downstream of the heating orcooling element and means (52) to determine a temperature difference inthe flowing substance upstream and downstream of the heating or coolingelement, flow control means comprising flow measurement means (61) formeasuring a mass flow characteristic and flow correction means (62) forcorrecting for measured mass flow variations, and evaluation means (70)for evaluating a characterising feature of the flowing substancecomprising a function relating temperature differences measured on oneor more calibration substances to one or more characterising features ofthe flowing substance.
 2. The device according to claim 1, wherein thefunction comprises a data-base or a calibration equation.
 3. The deviceaccording to claim 1, further comprising a flow rate means for adjustingflow through the transport duct in a detection range of the temperaturedifference sensor.
 4. The device according to claim 3, wherein the flowmeasurement means (61) comprises a pressure measurement cell (90) formeasuring a pressure difference over the temperature difference sensor(50) and the flow correction means (62) comprises a pressure differencecontrol means (91) for maintaining a substantially constant pressuredifference over the temperature difference sensor.
 5. The deviceaccording to claim 3, wherein the flow measurement means (61) comprisesa pressure measurement cell (90) for measuring a pressure differenceover the temperature difference sensor(50) and the flow correction means(62) comprises a computing means for mathematically correcting themeasured temperature difference for a measured pressure differencevariation.
 6. The device according to claim 3 ,wherein the flowmeasurement means (61) comprises a mass flow sensor (110) measuring themass flow through the temperature difference sensor (50) and the flowcorrection means (62) comprises a mass flow control means (91) formaintaining a substantially constant mass flow through the temperaturedifference sensor (50).
 7. The device according to claim 3, wherein theflow measurement means (61) comprises a mass flow sensor (110) measuringthe mass flow through the sensor (50) and the flow correction means (62)comprises a computing means (111) for mathematically correcting themeasured temperature difference for a measured mass flow variation. 8.The device according to claim 3, wherein the flow measurement means (61)comprises a pressure measurement cell (90) for measuring a pressuredifference over the temperature difference sensor (50), and furthercomprising a temperature correction means to correct for an absolutetemperature variation in the flowing substance and a pressure correctionmeans to correct for absolute pressure variations in the flowingsubstance.
 9. The device according to claim 1, wherein the flowmeasurement means (61) comprises a pressure measurement cell (90) formeasuring a pressure difference over the temperature difference sensor(50) and the flow correction means (62) comprises a pressure differencecontrol means (91) for maintaining a substantially constant pressuredifference over the temperature difference sensor.
 10. The deviceaccording to claim 9, further comprising a temperature correction meansto correct for an absolute temperature variation in the flowingsubstance and a pressure correction means to correct for absolutepressure variations in the flowing substance.
 11. The device accordingto claim 1, wherein the flow measurement means (61) comprises a pressuremeasurement cell (90) for measuring a pressure difference over thetemperature difference sensor (50) and the flow correction means (62)comprises a computing means for mathematically correcting the measuredtemperature difference for a measured temperature difference variation.12. The device according to claim 11, further comprising a temperaturecorrection means to correct for an absolute temperature variation in theflowing substance and a pressure correction means to correct forabsolute pressure variations in the flowing substance.
 13. The deviceaccording to claim 1, wherein the flow measurement means(61) comprises amass flow sensor (110) measuring the mass flow though the temperaturedifference sensor (50) and the flow correction means (62) comprises amass flow control means (91) for maintaining a substantially constantmass flow through the temperature difference sensor (50).
 14. The deviceaccording to claim 13, wherein the mass flow sensor (110) comprises acoriolis, an ultrasonic or a sonic nozzle mass flow sensor.
 15. Thedevice according to claim 1, further comprising a temperature correctionmeans to correct for an absolute temperature variation in the flowingsubstance.
 16. The device according to claim 15, wherein the temperaturecorrection means comprises a temperature measurement means and acalculation means (122) for mathematical correction of the measuredtemperature difference over the sensor (50) for a measured absolutetemperature variation.
 17. The device according to claim 1, furthercomprising a pressure correction means to correct for an absolutepressure variation in the flowing substance.
 18. The device according toclaim 17, wherein the pressure correction means comprises a pressurecontrol for maintaining a substantially constant absolute pressure inthe flowing substance.
 19. The device according to claim 1, wherein theflow measurement means (61) comprises a pressure measurement cell (90)for measuring a pressure difference over the temperature differencesensor (50), and further comprising a temperature correction means tocorrect for an absolute temperature variation in the flowing substanceand a pressure correction means to correct for absolute pressurevariations in the flowing substance.
 20. The device according to claim1, wherein the transport duct (30) is mounted as a by-pass (140) on amain duct(141) in which a part of the substance flows from the main ductthrough the transport duct and sensor and back into the main duct. 21.The device according to claim 1, further comprising a switch (160) forswitching on and off the flow control means, wherein, in anoff-position, the device measures a flow rate of the flowing substancethrough the transport duct.
 22. The device according to claim 21,comprising a means for intermittently switching the switch to the on-and the off position and a means to control the flow rate of the flowingsubstance in the off-position of the switch at a level determined by acharacterising feature of the flowing substance determined in theon-position of the switch.
 23. A method of use of the device accordingto claim 1, comprising: flowing a substance through the device of claim1; and identifying the flowing substance.
 24. A method of use of adevice according to claim 1, comprising: flowing a fuel substancethrough the device of claim 1; and controlling the flow of the fuelsubstance to deliver a controlled heat of combustion.
 25. A method ofuse of a device according to claim 1, comprising: flowing a substancethrough the device of claim 1; and determining a heat capacity of theflowing substance.
 26. A method of use of a device according to claim 1,comprising: flowing a substance through the device of claim 1; andidentifying a source or supplier of the flowing substance.
 27. A devicefor the characterisation of a flowing substance (20) comprising: atransport duct (30) on which is mounted a heating or a cooling element(40): a temperature difference sensor (50) comprising a firsttemperature measurement cell (51) downstream of the heating or coolingelement and means (52) to determine a temperature difference in theflowing substance upstream and downstream of the heating or coolingelement; flow control means comprising flow measurement means (61) formeasuring a mass flow characteristic and flow correction means (62) forcorrecting for measured mass flow variations; evaluation means (70) forevaluating a characterising feature of the flowing substance comprisinga function relating temperature differences measured on one or morecalibration substances to one or more characterising features of theflowing substance; and the flow measurement means (61) comprises a massflow sensor (110) for measuring the mass flow through the sensor (50)and the flow correction means (62) comprises a computing means (111) formathematically correcting the measured temperature difference for ameasured mass flow variation.
 28. The device according to claim 27,wherein the mass flow sensor (110) comprises a coriolis, an ultrasonicor a sonic nozzle mass flow sensor.
 29. A method for thecharacterisation of a flowing substance comprising: locally heating orcooling a substance flowing through a transport duct by a heating orcooling element, determining a temperature difference in the flowingsubstance upstream and downstream of the heating or cooling element,controlling mass flow of the flowing substance in the transport duct bymeasuring a mass flow characteristic and correcting for measuredvariations in the mass flow characteristic, and comparing the measuredtemperature difference with corresponding temperature differencesmeasured on one or more calibration substances for evaluating acharacterising feature of the flowing substance.
 30. The methodaccording to claim 29, wherein the measured mass flow characteristiccomprises a pressure difference over the heating-or cooling element andfurther comprising correcting the mass flow for a measured pressuredifference variation to maintain a substantially constant pressuredifference over the element.
 31. The method according to claim 29,wherein the measured mass flow characteristic comprises a pressuredifference over the heating-or cooling element and further comprisingcorrecting mathematically the measured temperature difference for ameasured pressure difference variation.
 32. The method according toclaim 29, wherein the measured mass flow characteristic comprises a realmass flow rate in the transport duct measured with a mass flow sensorand further comprising correcting the mass flow rate for a measured massflow rate variation to maintain a substantially constant mass flow rate.33. The method according to claim 29, wherein the measured mass flowcharacteristic comprises a real mass flow rate in the transport ductmeasured with a mass flow sensor and further comprising correctingmathematically the measured temperature difference for a measured massflow rate variation.
 34. The method according to claim 29, wherein themeasured mass flow characteristic comprises a pressure difference overthe heating-or cooling element and further comprising correcting themass flow rate for the measured pressure difference variation tomaintain a substantially constant pressure difference over the element,and correcting for absolute temperature variations and absolute pressurevariations in the flowing substance.
 35. The method according to claim29, further comprising identifying the flowing substance by retrievingfrom a database the identity of the calibration substance with the bestcorresponding temperature difference measurement.
 36. The methodaccording to claim 29, wherein the flowing substance is a natural gasand further comprising: characterising the natural gas by measuring thetemperature difference; and retrieving from a database or function,relating temperature difference measurements of different natural gasseswith one or more characterising features of said natural gasses.
 37. Themethod according to claim 36, wherein the database or function comprisesthe heat of combustion of calibration gasses and further comprisingusing the temperature difference measurement to determine the heat ofcombustion of the gas.
 38. A method for a combustion of a combustiongas, comprising: characterizing the combustion gas according to methodclaim 37; and mixing the combustion gas with an oxygen containing gas ina mixing ratio based on the measured temperature difference for thecombustion.